1. Field of the Invention
The present invention relates to a fuel cell having circular disk-shaped electrolyte electrode assemblies interposed between disk-shaped separators. Each of the electrolyte electrode assemblies includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates), and the electrolyte electrode assembly and the separators make up a unit of fuel cell for generating electricity. A predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the anode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as hydrogen-containing gas or CO is supplied to the anode. The oxygen ions react with the hydrogen in the hydrogen-containing gas to produce H2O or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric current.
Generally, the solid oxide fuel cell is operated at a high temperature in the range from 800° C. to 1000° C. The solid oxide fuel cell utilizes the high temperature waste heat for internal reforming to produce the fuel gas, and for spinning a gas turbine to generate electricity. The solid oxide fuel cell is attractive as it has the highest efficiency in generating electricity in comparison with other types of fuel cells, and receiving growing attention for potential use in vehicles in addition to the applications in combination with the gas turbine.
Typically, a sealing member such as a glass ring is inserted between the membrane electrode assembly and separators for preventing leakage of the fuel gas and the oxygen-containing gas supplied to the anode and the cathode of the membrane electrode assembly. Therefore, the fuel cell has a complicated structure, and the overall dimension of the fuel cell stack formed by stacking a plurality of the fuel cell is large in the stacking direction. In particular, in the solid oxide fuel cell operated at a high temperature, the sealing member is likely to be damaged by heat, and the desired sealing performance may not be maintained reliably.
In an attempt to address the problem, for example, Japanese Laid-Open Patent Publication No. 11-16581 discloses a solid oxide fuel cell. Specifically, as shown in FIG. 11, the cell includes a separator 1 having opposite main surfaces 2a, 2b. A plurality of ribs 3a are provided radially on the main surface 2a, and a plurality of ribs 3b are provided radially on the main surface 2b. Grooves 4a, 4b extend from the outside to central regions on the main surfaces 2a, 2b of the separator 1, respectively. The groove 4a has a predetermined depth to place a fuel gas supply pipe 5 in the groove 4a, and the groove 4b has a predetermined depth to place an oxygen-containing gas supply pipe 6 in the groove 4b. The fuel gas supply pipe 5 and the oxygen-containing gas supply pipe 6 are almost received inside the grooves 4a, 4b of the separator 1. Each of the fuel gas supply pipe 5 and the oxygen-containing gas supply pipe 6 has a planar shape having a thin end portion.
In the fuel cell, the fuel gas supplied to the fuel gas supply pipe 5 flows toward the central region on the main surface 2a of the separator 1, and the oxygen-containing gas supplied to the oxygen-containing gas pipe 6 flows toward the central region on the main surface 2b of the separator 1. The fuel gas is supplied to an electrolyte electrode assembly (not shown) on the side of the main surface 2a, and flows outwardly from a central region of the electrolyte electrode assembly. The oxygen-containing gas is supplied to another electrolyte electrode assembly (not shown) on the side of main surface 2b, and flows outwardly from a central region of the other electrolyte electrode assembly (not shown).
In the prior art, the grooves 4a, 4b each having a predetermined depth extend from the outside to central regions on the surfaces 2a, 2b of the separator 1, respectively, for placing the fuel gas supply pipe 5 in the groove 4a, and placing the oxygen-containing gas supply pipe 6 in the groove 4b. Thus, the separator 1 is likely to be deformed or damaged due to heat stress or the like. The chemical reaction in the electrolyte electrode assemblies may not be performed uniformly due to the presence of the fuel gas supply pipe 5 and the oxygen-containing gas supply pipe 6. Further, when many cells are stacked to form the fuel cell stack, the dimension of the fuel cell stack in the stacking direction is large due to the thickness of the fuel gas supply pipe 5 and the oxygen-containing gas supply pipe 6.